The present invention relates to the machining of the bearing diameter, particularly the main-bearing diameters, as well as of the adjoining cheek-side surfaces on crankshafts and similar workpieces.
In the case of these workpieces, it is desired not only, in particular, to obtain the highest possible precision upon production both of the roundness and of the desired diameter, especially of bearing diameters but, in addition, to obtain a manufacture which is as cost-favorable as possible, as is true in the case of all mass-produced workpieces. In order to achieve these goals, which frequently diverge in their effects, the machining method which seemed optimal was in the past already selected, namely cutting, milling or grinding. In addition, it is also clear that frequent rechucking of the workpiece during the machining leads, due to slight differences upon the rechucking, to inaccuracies in manufacture or at least to an increase in the time required for the next machining steps, so that complete machining in a single chucking is desirable.
In addition to the machining methods indicated above, so-called turning-broaching is also already known from German Application P 35 23 274; in it a disk-shaped broaching tool which has cutting edges on its circumference rotates slowly during the machining with respect to the rapidly rotating workpiece and machines the circumferential surface thereof. In this connection, the infeed movement can be effected in the manner that the individual cutting edges have an increasing distance from the center point of the turning-broaching tool on the circumferential surface of said tool. The infeed movement can, however, also be effected in the manner that all cutting edges are at the same distance from the center of the tool and the infeed movement is effected by a radial movement of the entire tool towards the workpiece. By this so-called turning-broaching, greater precision than with normal turning can be obtained upon the production of the bearing diameters. To be sure, the number of cutting edges of the turning-broaching tool is limited by the circumference of the tool, so that only limited radial oversizes of the workpiece can be removed.
Therefore, a combination of turning and turning-broaching, carried out on a single machine tool, has gained acceptance recently for the finish-machining of crankshafts. In this connection, both cutting edges used for the plunge-cut turning and cutting edges used for the turning-broaching are present on the tool, which is swingable around an axis parallel to the longitudinal axis of the workpiece. In the simplest case, a plunging-turning edge which has the width of the entire bearing diameter to be machined is present on this tool as well as a turning-broaching cutting edge which also has the full width of the bearing diameter to be produced. With the plunging turning tool--which in practice, is normally narrower than the length of the bearing diameter in order to avoid chattering--the bearing diameter is, first of all, cut down from the original high oversize to a very small oversize on the order of about 1/10 mm. The tool is then withdrawn from the workpiece and, while maintaining a high speed of rotation of the workpiece on the order of about 1000 rpm, the turning-broaching cutting edge of the swingable tool is swung in an arcuate path into the bearing diameter to be produced, the swinging movement being of such a speed that the turning-broaching cutting edge remains in engagement for more than one and generally for about two revolutions of the workpiece. With this turning-broaching machining, the bearing diameter of the crankshaft is completely machined to such an extent that pre-grinding can be dispensed with so that only finish-grinding is effected.
The machining of the side surfaces of the cheeks, which is effected by surfacing with a lathe tool, is effected before said machining.
In addition, recesses or even undercuts, etc., which are, for instance, required in the cheek side surfaces in the vicinity of the bearing diameter, must be produced by plunge feeding or turning-broaching, etc. The tools for this are also integrated in the swingable tool.
Although a faster advance of the machining, particularly upon the plunge-cutting, can be obtained by means of traditional turning than upon a turning-broaching which provides a good surface--particularly upon the removal of large amounts of material--milling is usually even faster but, to be sure, it results in a substantially poorer surface than turning-broaching or normal turning.